State-of-the-art electric energy conversion relies on a three-phase power network with alternating currents (AC) at 50 Hz or 60 Hz frequency and a voltage levels ranging from several hundreds of Volts to hundreds of thousands of Volts. The conversion of rotating mechanical energy into electric energy and vice versa is done by generators and by motors respectively. Those rotating machines can be divided into asynchronous and synchronous apparatuses. The three-phase windings of such machines comprise Copper conductors which require groundwall insulation especially in the slot section. When increasing the rated power output of such a machine, the voltage level increases so as to minimise use of material and to maximise efficiency.
Machines with a voltage level higher than 1000V require special field grading measures in the slot section in addition to the groundwall insulation. A slot corona protection is necessary to control the capacitive currents on both sides of the groundwall insulation and the axial voltage induced by the rotating magnetic field. A slot corona protection therefore establishes an electrode on the outer surface of the winding. Its surface resistance of 100Ω to 10 000Ω ensures a continuous electric connection of the slot corona protection to the grounded slot wall and limits the axial current flow. In addition, at the end winding a second type of field grading technology becomes necessary in order to control the voltage drop from the grounded slot section to the end connections. The end connections may even be on high-voltage potential. The commonly used solution for the end winding corona protection employs a resistive field grading material with a nonlinear resistance characteristic.
Both the end winding corona protection and the slot corona protection are designed to control surface currents in a way that no visible discharges (corona, arcing) occur and that safe and long-term operation of the machine is achieved. These systems are well known by each machine supplier and they are key technologies in view of machine design. They have been developed and have been optimised since the start of the electric energy conversion with high-voltage rotating machines. End winding and slot corona protection systems are well-defined for AC sinus voltages ranging from 16⅔ Hz to 60 Hz.
Synchronous rotating machines generate the magnetic field through rotor pole windings. The number of rotor poles and the frequency of the stator magnetic field defines the number revolutions per minutes (rpm) of the rotating machine.
With the advent of new power converters, the rotors of motors and of generators can be supplied with (sinusoidal) currents and voltages of varying frequency and phase angles. Accordingly, the number of revolutions per minute can now vary widely. Depending on the type of power converter either the current or the voltage shape may no longer be sinusoidal. The shapes of current or of voltage are instead determined by pulse width modulation (PWM).
Pulse width modulation involves switching between different voltage levels and results in rapid changes of voltage (high dU/dt). Consequently, capacitive currents and voltages will be much higher compared to sinus voltages at 50 Hz or at 60 Hz. In addition, the switching frequency is typically in the range of several hundred Hz. Due to these challenges the design of the groundwall insulation and also of the slot corona protection and of the end winding corona protection changes. New designs and technical developments are necessary to meet these requirements.
In pump storage plants (PSP) the standard synchronous machine can be replaced by a double-fed asynchronous machine. This type of machine allows for variable speed operation. The rotating part of the standard synchronous machine is in this case replaced by a three-phase high-voltage (at around 4-5 kV) winding supplied by a power converter. Voltage changes then occur at dU/dt≈1 kV/μs and switching frequencies of several hundred Hz.
The standard design for supporting the windings generally employs glass fibre cords and plates. For the rotor winding of double fed asynchronous machines, the high mechanical stress due to the centrifugal forces and vibrations requires the use of steel materials for mechanical support. This electric conductivity of these support materials results in new electric issues in the end winding area.
The present disclosure is oriented towards providing the aforementioned needs and towards overcoming the aforementioned difficulties.